The surface recording densities of magnetic storage devices such as hard disk drives have been increasing, necessitating the purchase and utilization of large numbers of expensive testing devices (testers) in order to select magnetic heads that are capable of achieving these high recording densities. Meanwhile, the price of magnetic storage devices has decreased, and the reduction of manufacturing costs has become an important issue that the manufacturers of magnetic storage device must address.
In order to address these trends, one approach of switching over from exhaustive testing to sampled testing in the selection tests which are performed on magnetic heads before they are transported from the magnetic head manufacturing plant to the magnetic storage device assembly plant has been examined, which can reduce the number of magnetic head testing devices used and may result in reducing overall manufacturing costs.
Similar issues exist in semiconductor manufacturing plants as well as in magnetic head manufacturing slants. For example, in Japanese Unexamined Patent Application Publication No. H8-274139, in order to reduce the time required for testing of semiconductor chips, only chips formed at predetermined sampling positions are tested instead of testing all the chips formed on the wafer surface, and the decision whether or not to test the other chips is then made based on the results of these tests. A criterion used in making this decision is the yield for each wafer; if a wafer has a high yield, then the sampling test results are relied upon and the chips are shipped without further testing.
In Japanese Unexamined Patent Application Publication No. H8-274139, a wafer with a high yield is trusted on the basis of the sampling tests alone, so all the semiconductor chips formed on the surface of this wafer are assumed to be good and are shipped. However, it is not possible to avoid having a considerable quantity of defective parts included among the shipped chips.
When sampling tests are applied to magnetic heads, it is necessary to quantify the proportion of defective parts included among the magnetic heads shipped from the magnetic head manufacturing plant to the magnetic storage device assembly plant, and to determine the unit cost per magnetic head based on the proportion of defective parts. When the proportion of defective parts increases, the failure rate of the assembled magnetic storage devices becomes higher, leading to higher manufacturing costs for the magnetic storage devices. On the other hand, subjecting the magnetic heads to sampling tests makes it possible to reduce the magnetic head manufacturing costs compared with heads that are subjected to exhaustive tests, thus allowing the unit cost per magnetic head to be reduced. To properly manage this trade-off relationship, it is necessary to reduce the defect inclusion ratio as much as possible, and to adjust this ratio to an appropriate value.
When magnetic heads are subjected to sampling tests, the magnetic head test results are fed forward as disclosed in Japanese Unexamined Patent Application Publication No. 2009-205774 and Japanese Unexamined Patent Application Publication No. 2009-211758, and it becomes difficult or impossible to apply control techniques to the magnetic storage device manufacturing process.
Therefore, testing and manufacturing methods for magnetic heads which alleviate the problems associated with prior art techniques would be very beneficial.